1. Field of the Invention
This invention relates to a process for producing toner particles of toners used in processes of rendering latent images visible and in toner jet recording processes, and a system for producing such toner particles.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691 and so forth are conventionally known as electrophotography. In general, copied images are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by the action of heat, pressure or solvent vapor. As methods for developing electrostatic latent images by the use of toners or methods for fixing toner images, a variety of methods have been proposed, and methods suited for the corresponding image-forming processes are employed.
Toners used for such purpose have commonly been produced by melt-kneading colorants such as dyes and/or pigments into thermoplastic resins to effect uniform dispersion, followed by pulverization and classification to produce toners having the desired particle diameters.
Reasonably good toners can be produced by such a production method, but there is a certain limit, i.e., a limit to the range in which toner materials are selected. For example, resin-colorant dispersions must be brittle enough to be pulverizable by means of economically available production apparatus. However, resin-colorant dispersions made brittle in order to meet such a requirement tend to result in a broad particle size range of the particles formed when actually pulverized at a high speed, especially causing such a problem that fine particles tend to be included in the particles in a relatively large proportion. Moreover, such highly brittle materials tend to be further pulverized or powdered when used in development in, e.g., copying machines. Also, in this method, it is difficult to perfectly uniformly disperse solid fine particles of colorants and so forth in the resin, and, depending on the degree of their dispersion, toners may cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency. Accordingly, care must be taken when they are dispersed. Also, colorants may come bare at rupture sections of toner particles, and may cause fluctuations in developing performance of toners.
Meanwhile, in order to overcome the problems of the toners produced by such pulverization, various polymerization toners and methods of producing such toners are proposed, including toners produced by suspension polymerization as disclosed in Japanese Patent Publications No. 36-10231, No. 43-10799 and No. 51-14895. For example, in the suspension polymerization, a polymerizable monomer, a colorant and a polymerization initiator, and also optionally a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a monomer composition. Thereafter, this monomer composition is dispersed in a continuous phase, e.g., an aqueous medium, containing a dispersion stabilizer, by means of a suitable agitator, and is simultaneously subjected to polymerization to obtain toner particles having the desired particle diameters.
Since this method has no step of pulverization at all, the toner particles are not required to be brittle, and hence soft materials can be used. Also, since it is possible to omit the step of classification, this method is greatly effective for cost reduction on account of energy saving, reduction of production time, improvements in process yield and so forth.
Toner itself is also required to be made multifunctional because copying machines and printers are made to satisfy demands for high-image-quality, full-color and energy-saving in recent years. For example, in order to make toner particles with finer particle diameters so as to be adaptable to high-resolution digital systems corresponding to higher image quality, to improve the transparency of OHP images corresponding to full-color image formation and to make toners fixable at a lower energy saving temperature, toners are required to contain low-softening materials and to have toner particle shapes effective for improving transfer efficiency to transfer materials. As a means for meeting such requirements, the toners produced by polymerization are useful.
On the other hand, the polymerization causes an increase in viscosity of polymerization systems with progress of polymerization in its reaction form inclusive of that for polymerization toners, to make it difficult for radicals and polymerizable monomers to move, so that unreacted polymerizable monomer components tend to remain in a large quantity. Especially in the case of suspension polymerization toners, components having a possibility of inhibiting polymerization reaction as exemplified by dyes, pigments (in particular, carbon black), charge control agents and magnetic materials are present in polymerizable monomer systems in a large quantity in addition to the polymerizable monomers, and hence the unreacted polymerizable monomers much more tend to remain.
Then, where any components acting as solvents for binder resins without limitation to the polymerizable monomers are present in such toner particles, they may lower the fluidity of toner to make image quality poor and besides cause a lowering of anti-blocking properties. Besides performances which correlate directly as those of toners, especially when organic semiconductors are used as photosensitive members, problems caused by phenomena of deterioration of photosensitive members as exemplified by memory ghost and blurred images may occur in addition to a phenomenon of melt-adhesion of toner to photosensitive drums. In addition to such matters concerning the performances of products, there is such a problem that the polymerizable monomer components volatilize at the time of fixing to give off a bad smell.
To improve the matters stated above, it is proposed as disclosed in Japanese Patent Application Laid-open No. 7-92736 that any unreacted polymerizable monomers present in toner particles are reduced to a residue of 500 ppm or less to bring about the effect of more improving image quality.
In addition, as copying machines and printers are made compact and personal, restrictions are more placed on apparatus and a greater load is imposed on the above problems. Also, there is an increasing interest in environment, and it is demanded to reduce VOC (volatile organic compounds) arising from toner particles, generated at the time of, e.g., fixing. Accordingly, the unreacted polymerizable monomers present in toner particles may preferably be reduced to a residue of 100 ppm or less.
As methods by which the unreacted polymerizable monomers present in toner particles are reduced to a much smaller residue, any known means for accelerating the consumption of polymerizable monomers may be used which are used when binder resins are produced by polymerization. For example, methods of removing unreacted polymerizable monomers may include a method in which they are washed with a highly volatile organic solvent capable of dissolving toner binder resins but not dissolving polymerizable monomers and/or organic solvents; a method in which they are washed with an acid or an alkali; a method in which a solvent component which does not dissolve foaming agents and polymers is put into a polymer system and the resultant toner is made porous to enlarge the area where the inside polymerizable monomer and/or organic solvent components volatilize; and a method in which polymerizable monomer and/or organic solvent components are volatilized under dry conditions. Because it is difficult to select solvents, e.g., due to the point that toner constituents may dissolve out as a result of deterioration of toner encapsulation or the solvent may remain, most preferred is the method in which polymerizable monomer and/or organic solvent components are volatilized under dry conditions.
In conventional cases, from toner particles obtained after a suspension having completely undergone polymerization reaction has been solid-liquid separated, the volatile components are commonly removed by means of a flash dryer, a vacuum dryer or the like.
Where the volatile components are removed using the flash dryer alone, toner particles are dried while being dispersed in high-velocity hot-air streams and being simultaneously forwarded in parallel flow with respect to that streams, and wet colored polymer particles can continuously be fed into the high-velocity hot-air streams. Hence, the dryer is one having a very good efficiency. Since, however, the drying time is instantaneous, it has been difficult to remove unreacted polymerizable monomers.
As disclosed in Japanese Patent Application Laid-open No. 8-160662, a method is also proposed in which toner particles are vacuum-dried. This drying method has an advantage that the drying targets can be dried at a low temperature. However, since the inside of its system stands evacuated, the gaseous phase may stagnate to greatly lower the force of diffusing the volatile components. Hence, in order to remove water content by evaporation and thereafter remove unreacted polymerizable monomers, it takes a very long drying time.
As disclosed in Japanese Patent Application Laid-open No. 10-207122, a method is still also proposed in which toner particles are vacuum-dried while gas streams are injected. However, as disclosed in this publication, the use of inert gas such as nitrogen or air as a gas to be injected brings about an improvement in drying efficiency in view of the effect of carrier gas that is put forth to keep the volatile components from stagnating, compared with an instance where the gas merely effects vacuum drying, but, in the case of dry gas such as inert gas, the amount of heat the gas itself can have is so small that the gas may take heat off from the toner particles having been heated, resulting in a lowering of drying efficiency.
This prolongs drying time to make longer the heat history applied to the toner, to cause deformation of particles and mutual melt-adhesion of particles, so that powder lumps may occur to lower image characteristics.
Where the gas streams are not kept temperature-controlled by, e.g., heating, gas streams whose temperature has been lowered because of heat insulation and expansion in the course of gas feeding come to enter as they are, to more greately take heat off from toner particles, resulting in a more lowering of drying efficiency.
Moreover, in the case of the gas, any resistance due to diffusion which hinders evaporation (which, however, is less than the case when no carrier gas is used) is a main factor which determines the drying speed. Where, e.g., the gas flow rate is made higher in order to improve the drying speed, it is necessary to enhance the capacity of evacuation equipment (chiefly vacuum pumps), resulting in a very high production cost. This is more remarkable in a mass production scale.
As discussed above, this drying system leaves many problems in respect of efficiency, product quality and cost. Solution of such problems has been considered to be a subject imposed on many engineers.
An object of the present invention is to provide a process for producing toner particles and a production system therefor, having solved the problems discussed above.
More specifically, an object of the present invention is to provide a process for producing toner particles and a production system therefor by which volatile components present in toner particles obtained by polymerization can be removed uniformly and also in a short time.
Another object of the present invention is to provide a process, and a system, for producing toner particles which can form high quality images having no defects caused by any remaining volatile components.
Still another object of the present invention is to save energy and cost which are necessary to remove the volatile components.
The present invention provides a process for producing toner particles, comprising;
polymerizing in an aqueous dispersion medium a polymerizable monomer composition containing at least a polymerizable monomer and a colorant, to form colored polymer particles, followed by washing and then dehydration to obtain toner particles; and
feeding the toner particles into an evacuatable and heatable container to carry out vacuum heat treatment while introducing into the container an injection medium having a temperature lower than glass transition temperature Tg of the toner particles and selected from the group consisting of i) saturated steam, ii) superheated steam and iii) high-humidity air having an enthalpy of 2,500 kJ/kg (dry air) or higher.
The present invention also provides a system for producing toner particles, which comprises an apparatus comprising;
a means for polymerizing in an aqueous dispersion medium a polymerizable monomer composition containing at least a polymerizable monomer and a colorant, to form colored polymer particles, followed by washing and then dehydration to obtain toner particles; and
a means for feeding the toner particles into an evacuatable and heatable to carry out vacuum heat treatment while introducing into the container an injection medium selected from the group consisting of i) saturated steam, ii) superheated steam and iii) high-humidity air having an enthalpy of 2,500 kJ/kg (dry air) or higher;
the vacuum heat treatment being performed while detecting temperature A of the injection medium and controlling the temperature A so as to fulfill the following condition:
30xc2x0 C. less than A less than glass transition temperature Tg of toner particles. 